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Syntax

undefined

Description

undefined is a property of the global object; i.e., it is a variable in global scope. The initial value of undefined is the primitive value undefined.

In modern browsers (JavaScript 1.8.5 / Firefox 4+), undefined is a non-configurable, non-writable property per the ECMAScript 5 specification. Even when this is not the case, avoid overriding it.

A variable that has not been assigned a value is of type undefined. A method or statement also returns undefined if the variable that is being evaluated does not have an assigned value. A function returns undefined if a value was not returned.

While it is possible to use it as an identifier (variable name) in any scope other than the global scope (because undefined is not a reserved word), doing so is a very bad idea that will make your code difficult to maintain and debug.

Examples

Strict equality and undefined

You can use undefined and the strict equality and inequality operators to determine whether a variable has a value. In the following code, the variable x is not initialized, and the if statement evaluates to true.

Note: The strict equality operator rather than the standard equality operator must be used here, because x == undefined also checks whether x is null, while strict equality doesn't. null is not equivalent to undefined. See comparison operators for details.

However, this kind of technique should be avoided. JavaScript is a statically scoped language, so knowing if a variable is declared can be read by seeing whether it is declared in an enclosing context. The only exception is the global scope, but the global scope is bound to the global object, so checking the existence of a variable in the global context can be done by checking the existence of a property on the global object (using the in operator, for instance).